Support graceful connection close

RFC 7540 section 6.8 seems to indicate that traffic can continue flowing in both directions after a GOAWAY frame, with the only restriction being that the recipient can't open new streams after receiving the GOAWAY:

   A GOAWAY frame might not immediately precede closing of the
   connection; a receiver of a GOAWAY that has no more use for the
   connection SHOULD still send a GOAWAY frame before terminating the
   connection.
...
   Activity on streams numbered lower or equal to the last stream
   identifier might still complete successfully.  The sender of a GOAWAY
   frame might gracefully shut down a connection by sending a GOAWAY
   frame, maintaining the connection in an "open" state until all in-
   progress streams complete.

But, h2's connection state machine immediately transitions to CLOSED upon sending or receiving a GOAWAY, effectively prohibiting any further traffic. Would you be interested in a patch to fix this, or is there some disconnect I'm missing between the wording of the RFC and actual implementations?

まず、RFC 7540 のセクション 6.8 と併せて h2 の接続状態マシンを確認し、GOAWAY の送信または受信後の遷移に焦点を当てます。現在、さらなるトラフィックが禁止されていることを再現し、期待される動作を特定します。作業完了の条件は、進行中のストリームは完了できる一方で、新しいストリームは禁止されたままとなり、両方向を対象とするテストが含まれていることです。